Transcript Lecture 21: Networking

CS162
Operating Systems and
Systems Programming
Lecture 21
Networking
November 14, 2005
Prof. John Kubiatowicz
http://inst.eecs.berkeley.edu/~cs162
Review: File System Caching
• Key Idea: Exploit locality by caching data in memory
– Name translations: Mapping from pathsinodes
– Disk blocks: Mapping from block addressdisk content
• Buffer Cache: Memory used to cache kernel resources,
including disk blocks and name translations
– Can contain “dirty” blocks (blocks yet on disk)
• Read Ahead Prefetching: fetch sequential blocks early
– Key Idea: exploit fact that most common file access is
sequential by prefetching subsequent disk blocks ahead of
current read request (if they are not already in memory)
• Delayed Writes: Writes to files not immediately sent
out to disk
– Instead, write() copies data from user space buffer to
kernel buffer (in cache)
– Flushed to disk periodically (e.g. in UNIX, every 30 sec)
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Review: Networking Definitions
• Network: physical connection that allows two computers
to communicate
• Packet: unit of transfer, sequence of bits carried over
the network
– Network carries packets from on CPU to another
– Destination gets interrupt when packet arrives
• Protocol: agreement between two parties as to how
information is to be transmitted
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Goals for Today
• Networking
– Broadcast Networks
– Point-to-Point Networks
– Network Protocols
Note: Some slides and/or pictures in the following are
adapted from slides ©2005 Silberschatz, Galvin, and Gagne
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Broadcast Networks
• Broadcast Network: Shared Communication Medium
Processor
I/O
Device
I/O
Device
I/O
Device
Memory
– Shared Medium can be a set of wires
Internet
» Inside a computer, this is called a bus
» All devices simultaneously connected to devices
– Originally, Ethernet was a broadcast network
» All computers on local subnet connected to one another
– More examples (wireless: medium is air): cellular phones,
GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO
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Broadcast Networks Details
Body
(Data)
Header
(Dest:2)
ID:1
(ignore)
Message
ID:3
(sender)
ID:4
(ignore)
ID:2
(receive)
• Delivery: When you broadcast a packet, how does a
receiver know who it is for? (packet goes to everyone!)
– Put header on front of packet: [ Destination | Packet ]
– Everyone gets packet, discards if not the target
– In Ethernet, this check is done in hardware
» No OS interrupt if not for particular destination
– This is layering: we’re going to build complex network
protocols by layering on top of the packet
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Broadcast Network Arbitration
• Arbitration: Act of negotiating use of shared medium
– What if two senders try to broadcast at same time?
– Concurrent activity but can’t use shared memory to
coordinate!
• Aloha network (70’s): packet radio within Hawaii
– Blind broadcast, with checksum at end of
packet. If received correctly (not garbled),
send back an acknowledgement. If not
received correctly, discard.
» Need checksum anyway – in case airplane
flies overhead
– Sender waits for a while, and if doesn’t
get an acknowledgement, re-transmits.
– If two senders try to send at same time, both get
garbled, both simply re-send later.
– Problem: Stability: what if load increases?
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» More collisions  less gets through more resent  more
load…  More collisions…
» Unfortunately: some sender may have started in clear, get
scrambled without finishing
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Carrier Sense, Multiple Access/Collision Detection
• Ethernet (early 80’s): first practical local area network
– It is the most common LAN for UNIX, PC, and Mac
– Use wire instead of radio, but still broadcast medium
• Key advance was in arbitration called CSMA/CD:
Carrier sense, multiple access/collision detection
– Carrier Sense: don’t send unless idle
» Don’t mess up communications already in process
– Collision Detect: sender checks if packet trampled.
» If so, abort, wait, and retry.
– Backoff Scheme: Choose wait time before trying again
• How long to wait after trying to send and failing?
– What if everyone waits the same length of time? Then,
they all collide again at some time!
– Must find way to break up shared behavior with nothing
more than shared communication channel
• Adaptive randomized waiting strategy:
– Adaptive and Random: First time, pick random wait time
with some initial mean. If collide again, pick random value
from bigger mean wait time. Etc.
– Randomness is important to decouple colliding senders
– Scheme figures out how many people are trying to send!
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Point-to-point networks
Router
Internet
Switch
• Why have a shared bus at all? Why not simplify and
only have point-to-point links + routers/switches?
– Didn’t used to be cost-effective
– Now, easy to make high-speed switches and routers that
can forward packets from a sender to a receiver.
• Point-to-point network: a network in which every
physical wire is connected to only two computers
• Switch: a bridge that transforms a shared-bus
(broadcast) configuration into a point-to-point network.
• Router: a device that acts as a junction between two
networks to transfer data packets among them.
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Point-to-Point Networks Discussion
• Advantages:
– Higher link performance
» Can drive point-to-point link faster than broadcast link
since less capacitance/less echoes (from impedance
mismatches)
– Greater aggregate bandwidth than broadcast link
» Can have multiple senders at once
– Can add capacity incrementally
» Add more links/switches to get more capacity
– Better fault tolerance (as in the Internet)
– Lower Latency
» No arbitration to send, although need buffer in the switch
• Disadvantages:
– More expensive than having everyone share broadcast link
– However, technology costs now much cheaper
• Examples
– ATM (asynchronous transfer mode)
» The first commercial point-to-point LAN
» Inspiration taken from telephone network
– Switched Ethernet
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» Same packet format and signaling as broadcast Ethernet,
but only two machines on each ethernet.
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Administrivia
• My office hours
– New office hour: Thursday 2:30-3:30
• MIDTERM II: Wednesday November 30th
– 5:30-8:30pm, 10 Evans
– All material from last midterm and up to Monday 11/28
– Includes virtual memory
• Final Exam
– December 17th, 12:30 – 3:30, 220 Hearst Gym
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Point-to-Point Network design
Queue
Inputs
Crossbar
Queue
Queue
Queue
Queue
Queue
Queue
Queue
Outputs
Control
(processor)
• Switches look like computers: inputs, memory, outputs
– In fact probably contains a processor
• Function of switch is to forward packet to output that
gets it closer to destination
• Can build big crossbar by combining smaller switches
Switch
3
Switch
2
Switch
1
• Can perform broadcast if necessary
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Flow control options
A,B
C
D
A
B,C,D
• What if everyone sends to the same output?
– Congestion—packets don’t flow at full rate
• In general, what if buffers fill up?
– Need flow control policy
• Option 1: no flow control. Packets get dropped if
they arrive and there’s no space
– If someone sends a lot, they are given buffers and
packets from other senders are dropped
– Internet actually works this way
• Option 2: Flow control between switches
– When buffer fills, stop inflow of packets
– Problem: what if path from source to destination is
completely unused, but goes through some switch that
has buffers filled up with unrelated traffic?
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Flow Control (con’t)
• Option 3: Per-flow flow control.
– Allocate a separate set of buffers to each end-toend stream and use separate “don’t send me more”
control on each end-to-end stream
aaaa
ababab
acbcac
bbbb
cccc
dddd
dadcdbdc
• Problem: fairness
– Throughput of each stream is entirely dependent on
topology, and relationship to bottleneck
• Automobile Analogy
– At traffic jam, one strategy is merge closest to the
bottleneck
» Why people get off at one exit, drive 50 feet, merge
back into flow
» Ends up slowing everybody else a huge emount
– Also why have control lights at on-ramps
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» Try to keep from injecting more cars than capacity of
road (and thus avoid congestion)
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The Internet Protocol: “IP”
• The Internet is a large network of computers spread
across the globe
– According to the Internet Systems Consortium, there
were over 353 million computers as of July 2005
– In principle, every host can speak with every other one
under the right circumstances
• IP Packet: a network packet on the internet
• IP Address: a 32-bit integer used as the destination
of an IP packet
– Often written as four dot-separated integers, with each
integer from 0—255 (thus representing 8x4=32 bits)
– Example CS file server is: 169.229.60.83  0xA9E53C53
• Internet Host: a computer connected to the Internet
– Host has one or more IP addresses used for routing
» Some of these may be private and unavailable for routing
– Not every computer has a unique IP address
» Groups of machines may share a single IP address
» In this case, machines have private addresses behind a
“Network Address Translation” (NAT) gateway
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Address ranges and subnets
• Address Ranges in IP
– IP address space divided into prefix-delimited ranges:
» NN.0.0.0/8:
Class A (10.0.0.0 private)
» NN.NN.0.0/16:
Class B (172.[16-31].0.0 private)
» NN.NN.NN.0/24: Class C (192.168.xx.0 private)
– Such ranges often owned by organizations
• Subnet: A network connecting a set of hosts with
related destination addresses
– With IP, all the addresses in subnet are related by a
prefix of bits
» The number of matching bits is called the “mask” and can
be expressed as a single value (e.g. 24) or a set of ones in
a 32-bit value (e.g. 255.255.255.0)
– A subnet is identified by 32-bit value, with the bits
which differ set to zero, followed by a slash and a mask
» E.g.: 128.32.131.0/24 designates a subnet in which all the
addresses look like 128.32.131.XX
» Same subnet: 128.32.131.0/255.255.255.0
• Difference between subnet and complete network range
– Subnet is always a subset of address range
– Once, subnet meant single physical broadcast wire; now,
less clear exactly what it means (virtualized by switches)
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Hierarchical Networking: The Internet
• How can we build a network with millions of hosts?
– Hierarchy! Not every host connected to every other one
– Use a network of Routers to connect subnets together
» Routing is often by prefix: e.g. first router matches first
8 bits of address, next router matches more, etc.
Other
subnets
subnet1
Router
Transcontinental
Link
Router
subnet2
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Other
subnets
Router
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subnet3
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Simple Network Terminology
• Local-Area Network (LAN) – designed to cover small
geographical area.
–
–
–
–
Multiaccess bus, ring, or star network
Speed  10 – 1000 megabits/second
Broadcast is fast and cheap
In small organization, a LAN could consist of a single
subnet. In large organizations (like Berkeley), a LAN
contains many subnets.
• Wide-Area Network (WAN) – links geographically
separated sites
– Point-to-point connections over long-haul lines (often
leased from a phone company)
– Speed  1.544 – 45 megbits/second
– Broadcast usually requires multiple messages
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Routing
• Routing: the process of forwarding packets hop-by-hop
through routers to reach their destination
– Need more than just a destination address!
» Need a path
– Post Office Analogy:
» Destination address on each letter is not
sufficient to get it to the destination
» To get a letter from here to Florida, must route to local
post office, sorted and sent on plane to somewhere in
Florida, be routed to post office, sorted and sent with
carrier who knows where street and house is…
• Internet routing mechanism: routing tables
– Each router does table lookup to decide which link to use
to get packet closer to destination
– Don’t need 4 billion entries in table: routing is by subnet
– Could packets be sent in a loop? Yes, if tables incorrect
• Routing table contains:
– Destination address range  output link closer to
destination
– Default entry (for subnets without explicit entries)
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Setting up Routing Tables
• How do you set up routing tables?
– Internet has no centralized state!
» No single machine knows entire topology
» Topology constantly changing (faults, reconfiguration, etc)
– Need dynamic algorithm that acquires routing tables
» Ideally, have one entry per subnet or portion of address
» Could have “default” routes that send packets for unknown
subnets to a different router that has more information
• Possible algorithm for acquiring routing table
– Routing table has “cost” for each entry
» Includes number of hops to destination, congestion, etc.
» Entries for unknown subnets have infinite cost
– Neighbors periodically exchange routing tables
» If neighbor knows cheaper route to a subnet, replace your
entry with neighbors entry (+1 for hop to neighbor)
• In reality:
– Internet has networks of many different scales
– Different algorithms run at different scales
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Network Protocols
• Protocol: Agreement between two parties as to how
information is to be transmitted
– Example: system calls are the protocol between the
operating system and application
– Networking examples: many levels
» Physical level: mechanical and electrical network (e.g. how
are 0 and 1 represented)
» Link level: packet formats/error control (for instance, the
CSMA/CD protocol)
» Network level: network routing, addressing
» Transport Level: reliable message delivery
• Protocols on today’s Internet:
NFS
Transport
RPC
UDP
Network
Physical/Link
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WWW
e-mail
ssh
TCP
IP
Ethernet
ATM
Kubiatowicz CS162 ©UCB Fall 2005
Packet radio
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Network Layering
• Layering: building complex services from simpler ones
– Each layer provides services needed by higher layers by
utilizing services provided by lower layers
• The physical/link layer is pretty limited
– Packets are of limited size (called the “Maximum Transfer
Unit or MTU: often 200-1500 bytes in size)
– Routing is limited to within a physical link (wire) or perhaps
through a switch
• Our goal in the following is to show how to construct a
secure, ordered, message service routed to anywhere:
Physical Reality: Packets
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Limited Size
Unordered (sometimes)
Unreliable
Machine-to-machine
Only on local area net
Asynchronous
Kubiatowicz CS162
Insecure
Abstraction: Messages
Arbitrary Size
Ordered
Reliable
Process-to-process
Routed anywhere
Synchronous
©UCB Fall 2005
Secure
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Building a messaging service
• Handling Arbitrary Sized Messages:
– Must deal with limited physical packet size
– Split big message into smaller ones (called fragments)
» Must be reassembled at destination
– Checksum computed on each fragment or whole message
• Internet Protocol (IP): Must find way to send packets
to arbitrary destination in network
– Deliver messages unreliably (“best effort”) from one
machine in Internet to another
– Since intermediate links may have limited size, must be
able to fragment/reassemble packets on demand
– Includes 256 different “sub-protocols” build on top of IP
• Process to process communication
– Basic routing gets packets from machinemachine
– What we really want is routing from processprocess
» Example: ssh, email, ftp, web browsing
– Several IP protocols include notion of a “port”, which is a
16-bit identifier used in addition to IP address
» A communication channel (connection) defined by 4 items:
[source address, source port, dest address, dest port]
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Building a messaging service (con’t)
• UDP: The Unreliable Datagram Protocol
–
–
–
–
Datagram: an unreliable packet sent from sourcedest
UDP layered on top of basic IP (Call it UDP/IP)
Unreliable, unordered, user-to-user communication
Important aspect: low overhead!
» Often used for high-bandwidth video streams
» Many uses of UDP considered “anti-social” – none of the
“well-behaved” aspects of (say) TCP/IP
• Ordered Messages
– Ordered messages on top of unordered ones
» IP can reorder packets! P0,P1 might arrive as P1,P0
– How to fix this? Assign sequence numbers to packets
» 0,1,2,3,4…..
» If packets arrive out of order, reorder before delivering to
user application
» For instance, hold onto #3 until #2 arrives, etc.
– Sequence numbers are specific to particular connection
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Performance Considerations
• Before continue, need some performance metrics
– Overhead: CPU time to put packet on wire
– Throughput: Maximum number of bytes per second
» Depends on “wire speed”, but also limited by slowest router
(routing delay) or by congestion at routers
– Latency: time until first bit of packet arrives at receiver
» Raw transfer time + overhead at each routing hop
Router
LW1
LR1
Router
LW2
LR2
Lw3
• Contributions to Latency
– Wire latency: depends on speed of light on wire
» about 1.5 ns/foot
– Router latency: depends on internals of router
» Could be < 1 ms (for a good router)
» Question: can router handle full wire throughput?
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Sample Computations
• E.g.: Ethernet within Soda
– Latency: speed of light in wire is 1.5ns/foot, which
implies latency in building < 1 μs (if no routers in path)
– Throughput: 10-1000Mb/s
– Throughput delay: packet doesn’t arrive until all bits
» So: 4KB/100Mb/s = 0.3 milliseconds (same order as disk!)
• E.g.: ATM within Soda
– Latency (same as above, assuming no routing)
– Throughput: 155Mb/s
– Throughput delay: 4KB/155Mb/s = 200μ
• E.g.: ATM cross-country
– Latency (assuming no routing):
» 3000miles * 5000ft/mile  15 milliseconds
– How many bits could be in transit at same time?
» 15ms * 155Mb/s = 290KB
– In fact, BerkeleyMIT Latency ~ 90ms
» Implies 1.7MB in flight if routers have wire-speed
throughput
• Requirements for good performance:
– Local area: minimize overhead/improve bandwidth
– Wide area: keep pipeline full!
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Reliable Message Delivery: the Problem
• All physical networks can garble and/or drop packets
– Physical media: packet not transmitted/received
» If transmit close to maximum rate, get more throughput –
even if some packets get lost
» If transmit at lowest voltage such that error correction just
starts correcting errors, get best power/bit
– Congestion: no place to put incoming packet
»
»
»
»
Point-to-point network: insufficient queue at switch/router
Broadcast link: two host try to use same link
In any network: insufficient buffer space at destination
Rate mismatch: what if sender send faster than receiver
can process?
• Reliable Message Delivery
– Reliable messages on top of unreliable packets
– Need some way to make sure that packets actually make
it to receiver
» Every packet received at least once
» Every packet received only once
– Can combine with ordering: every packet received by
process at destination once and in order
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A
Using Acknowledgements
B
A
B
Timeout
• How to ensure transmission of packets?
– Detect garbling at receiver via checksum, discard if bad
– Receiver acknowledges (by sending “ack”) when packet
received properly at destination
– Timeout at sender: if no ack, retransmit
• Some questions:
– If the sender doesn’t get an ack, does that mean the
receiver didn’t get the original message?
» No
– What it ack gets dropped? Or if message gets delayed?
» Sender doesn’t get ack, retransmits. Receiver gets message
twice, acks each.
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How to deal with message duplication
• Solution: put sequence number in message to identify
re-transmitted packets
– Receiver checks for duplicate #’s; Discard if detected
• Requirements:
– Sender keeps copy of unack’ed messages
» Easy: only need to buffer messages
– Receiver tracks possible duplicate messages
» Hard: when ok to forget about received message?
• Simple solution: Alternating-bit protocol
A
– Send one message at a time; don’t send
next message until ack received
– Sender keeps last message; receiver
tracks sequence # of last message received
B
• Pros: simple, small overhead
• Con: Poor performance
– Wire can hold multiple messages; want to
fill up at (wire latency  throughput)
• Con: doesn’t work if network can delay
or duplicate messages arbitrarily
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Conclusion
• Network: physical connection that allows two
computers to communicate
– Packet: sequence of bits carried over the network
• Broadcast Network: Shared Communication Medium
– Transmitted packets sent to all receivers
– Arbitration: act of negotiating use of shared medium
» Ethernet: Carrier Sense, Multiple Access, Collision Detect
• Point-to-point network: a network in which every
physical wire is connected to only two computers
– Switch: a bridge that transforms a shared-bus
(broadcast) configuration into a point-to-point network.
• Protocol: Agreement between two parties as to how
information is to be transmitted
• Internet Protocol (IP)
– Used to route messages through routes across globe
– 32-bit addresses, 16-bit ports
• Reliable, Ordered, Arbitrary-sized Messaging:
– Built through protocol layering on top of unreliable,
limited-sized, non-ordered packet transmission links
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